#include <bits/stdc++.h>
using namespace std;

typedef long long ll;

const int N = 105;

struct node {
	int x, y, flg;

	node() {}
	node(int x, int y, int flg): x(x), y(y), flg(flg) {}
};

int x, y, p, q, ans = 1e9, f[N][N][2];

bitset <N> vis[N][2];

queue <node> Q;

inline void bfs(node s) {
	Q.push(s);
	vis[s.x][s.y][s.flg] = 1;
	while (!Q.empty()) {
		node u = Q.front();
		Q.pop();
		if (u.flg == 0) {
			int s = x - u.x, t = y - u.y;
			if (s && s + q < t) {
				continue;
			}
		} else {
			int s = u.x, t = u.y;
			if (s && s + q < t) {
				continue;
			}
		}
		int d = f[u.x][u.y][u.flg];
		if (u.x == 0) {
			ans = d;
			break;
		}
		if (u.flg == 0) {
			for (int i = 0; i <= u.x; ++i) {
				for (int j = 0; j <= u.y && i + j <= p; ++j) {
					int nx = u.x - i, ny = u.y - j;
					if (!vis[nx][ny][1]) {
						Q.push(node(nx, ny, 1));
						vis[nx][ny][1] = 1;
						f[nx][ny][1] = d + 1;
					}
				}
			}
		} else {
			for (int i = 0; i <= x - u.x; ++i) {
				for (int j = 0; j <= y - u.y && i + j <= p; ++j) {
					int nx = u.x + i, ny = u.y + j;
					if (!vis[nx][ny][0]) {
						Q.push(node(nx, ny, 0));
						vis[nx][ny][0] = 1;
						f[nx][ny][0] = d + 1;
					}
				}
			}
		}
	}
}

int main() {
	#ifdef LOCAL
		assert(freopen("test.in", "r", stdin));
		assert(freopen("test.out", "w", stdout));
	#endif
	ios::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
	cin >> x >> y >> p >> q;
	bfs(node(x, y, 0));
	if (ans == 1e9) {
		cout << "-1\n";
	} else {
		cout << ans << "\n";
	}
	return 0;
}